In　order　to　study　the　effect　of　nitrate　on　the　biological　phosphorus　removal,　the　anaerobic-anoxic-aerobic　(A2/O)　oxidation　ditch　(OD)　process　was　adopted　to　treat　the　municipal　wastewater　combined　with　the　captive　test　for　4　months,　meanwhile　the　operation　data　of　an　A2/O　oxidation　ditch　wastewater　treatment　plant　were　analyzed.　The　effect　of　nitrate　on　phosphorus　release　in　the　anaerobic　zone　and　in　the　secondary　clarifier　was　studied.　The　volume　of　the　whole　reactor　was　375　L.　The　result　indicated　that　with　the　effluent　NO3-of　the　OD　more　than　5.0　mg/L,　the　phosphorus　release　was　restrained　by　NO3-carried　by　the　returned　activated　sludge,　resulting　in　the　decrease　of　TP　removal　rate　with　the　increase　of　NO3-in　the　effluent.　When　NO3-in　the　effluent　was　less　than　5.0　mg/L,　the　TP　removal　rate　was　decreased　with　the　decrease　of　NO3-in　the　effluent　due　to　the　phosphorus　release　happened　in　the　low　NO3-condition　with　inner　carbon　resource　acting　as　an　electronic　acceptor.　The　captive　test　indicated　that　the　phosphorus　release　with　inner　carbon　resource　acting　as　the　electronic　acceptor　was　restrained　when　NO3-was　more　than　0.5　mg/L　in　the　secondary　clarifier,　while　phosphorus　was　released　with　the　inner　carbon　being　the　electronic　acceptor　when　NO3-was　less　than　.5　mg/L,　and　the　specific　phosphorus　release　rate　was　0.18-0.47　mg/(gVSS　·　h).　The　operation　result　of　the　wastewater　treatment　plant　indicated　that　retention　time　of　the　activated　sludge　in　the　secondary　clarifier　was　so　long　that　it　would　increase　the　effluent　TP　due　to　the　phosphorus　release　with　inner　carbon　resource.